The aims of this research proposal have been to investigate host responses toward transplanted tissue allografts, both those involved in the phenomenon of acute rejection by unmodified recipients and those in prolonging graft survival in enhanced animals. Our studies have been concerned primarily with assessment of the dynamic and changing relationships between cellular and humroral host mechanisms involved in these processes. Cellular responses have been studied by assessing activity of T lymphocyte populations and subpopulations, both those infiltrating the graft itself and those in peripheral lymphoid tissues. The role of thymocytes in acute rejection and enhancement has been pursued, as evidence of suppressor activity has been noted previously in enhanced animals. Patterns of migration of thymocytes and of T and B lymphocytes are being assessed by double radiolabelling experiments in both unmodified and enhanced graft recipients. Interaction between lymphocytes and macrophages in acute rejection and enhancement are under investigation.